The present invention concerns a process for recovering silver from lead solutions containing silver and, optionally copper and zinc, which lead solutions are obtained by means of hot chloride leaching with, for example, brine, with or, without chlorine being introduced; that is to say, the lead solutions are obtained from residues including flue dusts containing lead and silver and possibly copper, zinc, and iron.
In chemically working up lead-bearing complex concentrates in the wet way, there are obtained, as a rule, residues containing lead as their chief constituent. In most of these residues, in which the lead is often present as lead sulphate, there are contained varying amounts of silver the recovery of which is a precondition for working up the complex concentrates economically. Besides silver, there can also be present considerable amounts of other non-ferrous metals such as, for example, copper and zinc. The residues under consideration are leaching residues from the working up of the above-mentioned complex concentrates by means of, for example, sulphatizing chlorinating roasting or dead roasting, or by means of direct pressure leaching with or without a prior activation step.
In chemically working up zinc concentrates in the wet way by means of dead roasting and leaching, there are known to be obtained residues containing, besides zinc as their chief constituent, varying amounts of lead and silver, the so-called ZES (zinc electrolyis sludges). In the more recent processes -- jarosite and goethite processes -- the bulk of the lead and the silver remains in a silver-bearing lead sulphate leaching residue.
Furthermore, these are obtained in the pyrometallurgical working up flue dusts often containing lead and silver. In these flue dusts the lead is usually present as lead chloride.
For the purpose of recovering the silver and the lead from the above-mentioned and similar starting materials, which, with regard to the order of magnitude, may contain 2 to 25, preferably 10 to 25 % Pb and 30 to 1000, preferably 300 to 1000 g/t Ag and differing amounts of copper, zinc, cadmium, and other accompanying metals, it is known to subject these materials to leaching with chloride solutions, preferably with a hot acid sodium chloride solution. Salts, which are difficult to dissolve, such as, for example, lead sulphate and silver chloride, are dissolved as chlorocomplexes. With a chloride content in the solution of &gt; 150 g/l, there can be obtained &gt; 30 g pb/l and about 2 g Ag/l.
As is well known, it is necessary for the oxidation potential to be increased by means of chlorine gas being added, for example, if the non-ferrous metal content, not yet oxidized, is to be oxidized.
In this process, such minor constituents as copper, zinc, and cadmium are partially or completely dissolved, too. However, ferrites as might be present will not be dissolved, so that from starting materials with larger amounts of copper and/or of zinc fixed as ferrites (for example, the zinc in ZES*), high yields are obtained only of the lead and the silver. FNT * = residues from electrolytic zinc plants
As is also well known, the sodium chloride solution can be used again subsequent to the non-ferrous metals, dissolved in it, having been precipitated, in doing which the precipitation of the metals is effected by means of cooling the solution, by means of cementation processes or by means of precipitation with alkali or alkaline earth compounds, preferably with soda or lime.
By means of cementation of the lead solution with iron, there is obtained cement lead containing silver and, as the case may be, copper, with the tendency of the cement lead to cake together, however, causing technical difficulties in discharging the cementate from the reactors, for example.
In precipitating the lead with the aid of alkaline agents preferably with the aid of lime for economical reasons, there is obtained a basic lead chloride, also containing the silver and the copper, precipitated simultaneously. The lead, the silver, and the copper are jointly precipitated as basic carbonates in the same manner during precipitation with soda. The further treatment of these precipitates in accordance with well-known processes, such as, for example, fusion reduction to base bullion (subsequent, if need be, to a dechlorinating step) requires further refining for the purpose of separating the lead, the silver, and the copper, which is a further considerable burden to the economy of the recovery of the individual metals.